1. Field of the Invention
The present invention relates to a method and an apparatus for checking important documents, securities, paper money, cards, art objects, and other checking objects which should be checked for authenticity.
2. Description of the Related Art
A checking method and apparatus utilizing microwaves are conventionally known as measures for checking the authenticity of documents and the like, as described in U.S. Pat. No. 4,820,912. According to this prior art, microwaves are applied to a number of particles which are distributed at random in each document, and a proper digital mark responsive to a response microwave bundle are recorded in a suitable region of the document according to specific rules. In checking the authenticity of the document, microwaves are applied to the document, and the response microwave bundle is collated with the digital mark. The document is judged to be real when the microwave bundle and the mark agree with each other.
According to the prior art checking means based on the microwaves, however, measuring the response microwave bundle is susceptible to external noises, so that a satisfactory signal-to-noise ratio cannot be enjoyed. Used to oscillate the microwaves, moreover, the aforesaid prior art apparatus may possibly constitute a source of noises. Generally, furthermore, a microwave transmitter and receiver are large-sized, and entail high costs.